The 3E FRCS, the Old Grey Box, the Neverwinter Campaign Setting, and Ed Greenwood Presents: Elminster's Forgotten Realms have been my go to sourcebooks for as long as I've had them (some longer than others, obviously). And of course Volo's Guide to Cormyr. And then a zillion more honorable...
I think skilled play is about a DM knowing the players at the gaming table. Then it’s about finding and leveraging the mechanics in the system being played to enhance the table’s experience.
If the system doesn’t have those mechanics then it’s time to look outside the system or dream up some...